1. Field of the Invention
The invention relates to a mirror for systems for utilization of solar energy, with a substrate of low absorption flat glass and a rear highly reflective silver reflective layer, which is provided with a protective coating, a process for its manufacture, and its use.
2. Background Information
Such a mirror is known for example from German Laid Open Patent Application DE-A 30 04 353. The glass substrate described there is normal flat glass which is made especially thin to reduce energy absorption. In practice as an alternative to this, white or high transmission glass is used for the substrate, which possesses significantly lower iron content than normal float glass and thus higher energy transmission. With mirrors of such construction, reflection factors for solar energy of over 90% are achieved.
Another mirror is known from U.S. Pat. No. 4,511,618 entitled Laminated Reflective Panels, which corresponds to German Laid Open Patent Application DE-32 14 853 A 1, which relates to a laminated reflective panel comprising at least one mirror sheet which is less than 2.0 mm in thickness and which bears a reflective coating on its rear face. Each mirror sheet is laminated to a flat glass backing sheet which is thicker than the mirror sheet. The backing sheet is preferably made of float glass.
An article which relates to the optical properties of thin layers is found in "chemie-anlagen+verfahren" Heft 7 and 8/1969 in an article by Dr. Klaus Deutscher. This article relates to the optical properties of thin layers in regard to optical and opto-electronic devices This article further relates to thin layers used for the decrease of reflection and for the increase of reflection in the division of light in filters, masks and as polarizers built into such devices.
Mirrors of the generic type offer a series of advantages over other known solar mirrors. The use of the silver layer as reflective layer leads to efficiency levels when utilizing solar energy in solar heat systems which cannot be achieved with any other material. The disadvantage of the relatively high sensitivity of the silver layer to abrasion and corrosion is countered by the silver layer being located on the back of the substrate and being covered with a protective coating which usually comprises a copper layer as well as varnishing.
Alternatively or supplementary thereto, it is possible to provide in known fashion on the back of the silvered glass substrate an additional sheet of glass, metal, glass fibre reinforced plastic or the like, if this is required for protection of the silver layer or for stabilization of the mirror.
By using inorganic glass as substrate of the solar mirror, it is possible to achieve a service life of 20 years and more, even under extreme climatic conditions, the glass substrate--in contrast to the plastic substrate--not requiring any protection against the UV content of the solar radiation.
Mirrors of the aforementioned type are being used to an increasing extent for commercial utilization of solar energy in solar heat systems. One application takes the form of parabolic trough solar power stations, where cylindrically curved parabolic mirrors concentrate the solar energy onto tubes positioned along the focal line, in which circulates a heat transfer medium, which carries the heat absorbed to a generator station, where it is utilized for power generation.
Another application takes the form of solar tower power stations, where a number of plane mirrors direct the solar energy onto a "hot spot" on a tower, from whence the heat is dissipated via a high temperature heat transfer medium such as sodium and is also utilized for power generation.
The efficiency of solar heat systems is itself limited in optimum utilization of the solar energy for thermodynamic reasons. It would therefore be desirable to supply the solar rays directly to a utilization facility, that is to say without the diversion via process heat. A possibility to this end is offered by chemical processes. In the case of the so-called solar chemical utilization of solar energy, it is possible essentially to distinguish between three categories of chemical processes;
1. Generation of energy carrier media, e.g. hydrogen, synthesis gas, ammonia, methanol, aluminum.
2. Generation of primary materials, e.g. calcination of limestone, reduction of ores, desalination of water, production of carbon fibres, of nylon, of vitamin D.
3. Radiation induced reactions, e.g. the treatment of surfaces (alloy formation), decomposition of toxic substances, etc.
As with solar heat systems, it is necessary if possible with solar chemical systems to increase the energy density, for example by means of parabolically curved mirrors. It is important with the solar chemical processes mentioned for the reflectors to possess a high reflection factor, particularly in the short wave region of the solar spectrum. The reason for this is to be found in the fact that on the one hand, a series of chemical processes will only take place at all if a sufficiently high proportion of high energy, that is to say short wave radiation, is present. On the other hand, the thermodynamic quality of the electromagnetic radiation increases, the shorter the wavelength of the radiation is.
Attempts have already been made to provide solar mirrors for solar chemical processes which, in comparison with the rearsilvered glass mirrors of the generic type, possess an increased reflection factor in the UV region. In the case of these mirrors, aluminum is used for the reflective layer; this in contrast to silver possesses a sufficiently high reflection factor in the UV region. To prevent the UV content of the solar radiation being absorbed in the glass before even reaching the aluminum layer, it is necessary with these mirrors to apply the aluminum layer to the front of the substrate.
Such a configuration does in fact actually increase the reflection factor in the UV region. Locating the aluminum layer on the front reduces resistance to ageing and scratching however, even if the aluminum layer is covered with a dielectric protective layer. In addition, with the layer thicknesses usual and necessary, such a protective layer produces inter alia by interference undesirable reduction of the reflection factor of the aluminum layer in some regions of the spectrum. A further disadvantage is the lower overall solar reflection factor of aluminum, this amounting to several percent.